STOCKHOLM—Klarna said it will become the exclusive buy-now-pay-later provider at Walmart in the U.S., replacing rival Affirm as the Swedish fintech company nears a potential initial public offering.Stockholm-based Klarna said Monday that it will work with Walmart-backed consumer finance app OnePay to offer customers installment loans on purchases at the retailer.

Affirm shares are tumbling after rival Klarna, which is preparing to go public in the U.S., said it is now the exclusive provider of Buy Now, Pay Later loans for Walmart.

Klarna, which is a leading buy-now-pay-later provider (BNPL), has partnered with OnePay, a finance app backed by retail giant Walmart (WMT), in